Recently, there has been a strong demand to improve fuel efficiency to reduce CO2 emission of automobiles due to growing awareness of global environment conservation. This has led to a trend of reducing the necessary thickness of materials for automobile bodies by strengthening the materials to reduce the weight of automobiles. However, there is a concern that strengthening of a steel sheet is accompanied by a decrease in ductility and stretch flange formability. Therefore, there has been a demand for the development of a high-strength steel sheet having high ductility and high stretch flange formability. Moreover, from the viewpoint of the achievement of buckling resistance for realizing occupant protection in a side collision, the demand for a steel sheet with high yield ratio has also been growing. In addition, the need for a high-strength galvanized steel sheet has been growing as a material for parts in consideration of corrosion resistance.
To improve stretch flange formability, for example, Japanese Unexamined Patent Application Publication No. 2008-291314 discloses a high-strength galvannealed steel sheet having stable in material quality and excellent stretch flange formability which has the specified chemical composition and a multi-phase microstructure in which the area ratio and the grain size of a ferrite phase, the size and the amount of fine precipitates in a ferrite phase, and the area ratio of a retained austenite phase are specified, and a method for manufacturing the steel sheet. Moreover, to increase the stretch flange formability and the yield ratio of a high-strength steel sheet, Japanese Unexamined Patent Application Publication No. 2001-335892 discloses a high-strength hot-rolled steel sheet and a high-strength galvanized steel sheet having excellent formability which has the specified chemical composition and a multi-phase microstructure in which the hard phase microstructures of a martensite phase, a retained austenite phase and a bainite phase are finely dispersed in a ferrite matrix and a method for manufacturing the steel sheets. Furthermore, Japanese Unexamined Patent Application Publication No. 2008-274416 discloses a hot-rolled steel sheet having excellent fatigue property and stretch flange formability which has the specified chemical composition and a ferrite-based microstructure in which the ratio between Ti content and C content is controlled and a method for manufacturing the steel sheet.
However, as a main object of JP '314 is to improve stretch flange formability, an increase in ductility and yield ratio is not considered. Moreover, although an increase in ductility and yield ratio is considered in JP '892 and JP '416, the improvement in ductility is not investigated. Therefore, the development of a high-strength galvanized steel sheet having all of high ductility, high stretch flange formability and high yield ratio remains a problem.
In view of the situation described above, it could be helpful to provide a galvanized steel sheet with high yield ratio having not only high strength (tensile strength TS of 590 MPa or more), but also excellent ductility and stretch flange formability and a method for manufacturing the steel sheet.